MEMS parameter identification using modulated waveforms

ABSTRACT

A sensor system includes a microelectromechanical systems (MEMS) sensor, control circuit, signal evaluation circuitry, a digital to analog converter, signal filters, an amplifier, demodulation circuitry and memory. The system is configured to generate high and low-frequency signals, combine them, and provide the combined input signal to a MEMS sensor. The MEMS sensor is configured to provide a modulated output signal that is a function of the combined signal. The system is configured to demodulate and filter the modulated output signal, compare the demodulated, filtered signal with the input signal to determine amplitude and phase differences, and determine, based on the amplitude and phase differences, various parameters of the MEMS sensor. A method for determining MEMS sensor parameters is also provided.

This application is a divisional application of U.S. application Ser.No. 13/948,775, having a filing date of Jul. 23, 2013, having commoninventors, and having a common assignee, all of which is incorporated byreference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to microelectromechanicalsystems (MEMS) devices utilized in electronic systems. Morespecifically, the present invention relates to electrically testing andidentifying parameters of MEMS devices utilizing modulated waveforms.

BACKGROUND OF THE INVENTION

Microelectromechanical Systems (MEMS) sensors are widely used inapplications such as automotive electronics, inertial guidance systems,household appliances, consumer electronics, protection systems, and manyother industrial, scientific, engineering and portable systems. SuchMEMS sensors are used to sense a physical condition such as, forexample, acceleration, pressure, angular rotation, or temperature, andto provide an electrical signal representative of the sensed physicalcondition to the applications and/or systems employing the MEMS sensors.The applications and/or systems may utilize the information provided bythe MEMS sensor to perform calculations, make decisions, and/or takecertain actions based on the sensed physical condition.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures (not necessarily drawn to scale), whereinlike reference numbers refer to similar items throughout the Figures,and:

FIG. 1 shows a simplified side view of a MEMS sensor configured inaccordance with the teaching of an embodiment;

FIG. 2 shows a block diagram of a sensor parameter identification andevaluation system including the MEMS sensor of FIG. 1 configured inaccordance with the teaching of an embodiment;

FIG. 3 shows a block diagram of a sensor parameter identification andevaluation system configured in accordance with the teaching of analternative embodiment;

FIG. 4 shows a block diagram of a sensor parameter identification andevaluation system configured in accordance with the teaching of anotheralternative embodiment; and,

FIG. 5 shows a flow chart of a sensor parameter identification andevaluation method, according to an embodiment.

DETAILED DESCRIPTION

Capacitive-sensing MEMS designs are highly desirable for operation inacceleration, angular rotation, pressure environments and inminiaturized devices due to their relatively low cost. When subjected toacceleration, angular rotation, pressure, or some other externalstimulus to which the MEMS device is designed to be responsive,capacitive sensing MEMS devices provide a change in electricalcapacitance that corresponds to the magnitude of the applied stimulus.In other words, the electrical output at a given time of a MEMS devicecorresponds to the magnitude of the stimulus applied to that MEMS deviceat that given time. In this manner, by monitoring the electrical outputof a MEMS device, a system may determine the magnitude of externalstimuli applies to various MEMS devices (pressure, acceleration, angularrotation, etc.), and use that information to help determine what actionsthe system should take responsive to the stimuli. For example, anautomotive air bag system sensing a rapid deceleration of the automobilebased on the electrical output of a MEMS accelerometer device maydetermine that it is necessary to deploy an airbag in order to protect avehicle occupant. One common form of MEMS device is an accelerometer inthe form of a two layer capacitive transducer having a “teeter-totter”or “see saw” configuration. This commonly utilized transducer type usesa movable element or plate that rotates under z-axis acceleration abovea substrate. The accelerometer structure can measure two distinctcapacitances to determine differential or relative capacitance, andprovide that information as an output to the MEMS accelerometer. OtherMEMS devices designed to sense other applied stimuli may take on variousforms, provided that the output of the MEMS device is configured tocorrespond to the magnitude of the stimulus being monitored.

The electro-mechanical characteristics, also referred to as parameters,of each MEMS device may differ due to a variety of factors(manufacturing tolerances, slight differences in processing depending onwhere and when the MEMS device was manufactured, etc.). This means thatthe electrical output of one MEMS device responsive to a certainmagnitude of stimulus might be different from the electrical output of asecond MEMS device responsive to a stimulus of the very same magnitude.Because systems employing MEMS devices may use the electrical output tocalculate the extent of the stimulus, and may use the result of thatcalculation to determine whether or not to take a certain action (suchas, for example, deploying an airbag), it is important that parametersof MEMS devices be identified and evaluated such that a system employingMEMS sensors can be compensated (for offset) and calibrated (for gain)to correlate a given electrical output from the MEMS sensor to aspecific amount of applied stimulus. Typically, identification andevaluation of MEMS parameters, and calibration of the system employingthe MEMS sensors, occurs prior to shipment of the system employing theMEMS sensor. Parameter identification and evaluation may be accomplishedby applying the actual physical stimulus (for example, acceleration) tothe system, measuring the electrical response of the MEMS sensor, andstoring values representative of the MEMS parameters in the system,along with values representative of any “correction” or calibrationfactors that need to be applied to the electrical output of the MEMSsensor in light of the MEMS parameters. Identification of the MEMSparameters and application of correction or calibration factors to theMEMS output help to make sure that the MEMS sensor output corresponds tothe magnitude of the applied stimulus.

Although physically applying various stimuli to MEMS sensors, andsystems employing MEMS sensors, can serve to identify MEMS parametersand provide resulting calibration data such that the system can functionproperly, such physical testing can be expensive, time-consuming anddamaging to the system being tested. Furthermore, the need to physicallytest a variety of systems and applications employing MEMS sensors canrequire numerous, application-specific test stations to be designed andbuilt for each application to be tested, increasing the cost and timeassociated with such testing. In addition, although physical testingprior to shipment of MEMS sensors, and systems employing MEMS sensors,can provide parameter values that may be accurate at the time oftesting, such testing ignores the fact that over time (and as a resultof use and/or damage), the electro-mechanical parameters of the MEMSdevices may change, making the initial parameter values no longeraccurate, and possibly causing system malfunction. Finally, in the eventof a system malfunction, attempts to determine the cause of themalfunction can be complicated by requiring that the system employingthe MEMS device be physically removed from the end application (such as,for example, an automobile) so that the MEMS device can again bephysically tested MEMS parameters evaluated. Such a requirement can makemaintenance and repair of systems employing MEMS devicescost-prohibitive.

In one aspect, a system and method are provided for the electricaldetermination of MEMS device parameters utilizing modulatingelectromagnetic signals, without a need for applying physical forcesexternal to the MEMS device. In an additional aspect, a system andmethod are provided for testing, monitoring and evaluating MEMS deviceparameters utilizing modulating electromagnetic signals throughout thelifetime of those MEMS devices and while those devices remain locatedand functioning in systems employing the MEMS devices. Thus, in someembodiments, systems and methods achieving design objectives of low-costMEMS sensor testing, repeated identification and evaluation of MEMSsensor parameters throughout the life of the MEMS sensors, testing ofMEMS sensors without removal of the sensors from the application, andre-calibration of systems employing MEMS devices during the life of theMEMS devices are provided.

FIG. 1 shows a simplified side view of a MEMS sensor 10 configured inaccordance with the teaching of an embodiment. MEMS sensor 10 is anelectromechanical z-axis sensor configured with at least one moveablemechanical element 14. The moveable mechanical element 14 is aconductive metalloid plate physically coupled to a substrate 12 of MEMSsensor 10 by means of a mounting system. In an alternative embodiment,moveable mechanical element 14 is made of any material capable ofconducting electrical signals. The simplified diagram of FIG. 1 showsthe mounting system comprising a pedestal 19 which is physically coupledto the substrate 12 and a torsion bar 11 defining a flexure axis aboutwhich moveable mechanical element 14 may rotate, physically coupled topedestal 19 and moveable mechanical element 14. Torsion bar 11 andpedestal 19 are made of conductive material. Because moveable mechanicalelement 14, torsion bar 11, and pedestal 19 are all made of conductivematerial and coupled together, moveable mechanical element 14, torsionbar 11, and pedestal 19 are all at the same potential, such that anelectromagnetic signal applied to pedestal 19 will be conducted intomoveable mechanical element 14. Pedestal 19 and torsion bar 11 serve tosuspend moveable mechanical element 14 above and substantially parallelto the surface of substrate 12 in the absence of physical or otherforces applied to the MEMS sensor 10. Non-illustrated conductors areformed in the substrate 12 such that pedestal 19 is electrically coupledto a surface contact of MEMS sensor 10 located on a surface of substrate12 to permit electrical contact to circuitry external to MEMS sensor 10,and such that electromagnetic signals applied to the surface contact areconducted into pedestal 19. MEMS sensor 10 further comprises sensecontacts 18 and 9 made of electrically conducting material and coupledto the upper surface of substrate 12. Non-illustrated conductors areformed in the substrate 12 such that sense contacts 18 and 9 are eachelectrically coupled to surface contacts of MEMS sensor 10 located on asurface of substrate 12 to permit electrical contact to circuitryexternal to MEMS sensor 10, and such that electromagnetic signalsapplied to the surface contacts are conducted into sense contacts 18 and9, and such that electromagnetic signals created by movement ofmechanical element 14 relative to sense contacts 18 and 9 are conductedinto the surface contacts. The area between the upper surface of contact18 and the lower surface of moveable mechanical element 14 forms a firstcapacitor, and the area between the upper surface of contact 9 and thelower surface of moveable mechanical element 14 forms a secondcapacitor. Application of a voltage differential between the moveablemechanical element 14 and sense contact 18 may induce movement inmoveable mechanical element 14, and application of a voltagedifferential between the moveable mechanical element 14 and sensecontact 9 may induce movement in moveable mechanical element 14.Furthermore, physical movement of moveable mechanical element 14 causesa change in capacitance of the first capacitor and/or the secondcapacitor.

Moveable mechanical element 14 is configured to move (about the axisformed by torsion bar 11) relative to the surface of substrate 12 andsense contacts 18 and 9 of MEMS sensor 10 responsive to physical and/orelectrical stimuli applied to MEMS sensor 10 and/or moveable mechanicalelement 14. Moveable mechanical element 14 has a pre-determined range oftravel relative to the substrate 12. When moveable mechanical element 14is caused to move by a physical, electromagnetic, or other stimuli towhich moveable mechanical element 14 is exposed, an electromagneticsignal is conducted into sense contacts 18 and 9 of MEMS sensor 10, andinto their surface contacts. This electromagnetic signal variesaccording to the movement of the moveable mechanical element 14 relativeto sense contacts 18 and 9 within the predefined range of travel. In anembodiment, MEMS sensor 10 is configured such that, when the moveablemechanical element 14 of MEMS sensor 10 moves, it continuously providescapacitance values at sense contacts 18 and 9 that correspond to themagnitude of the motion of the moveable mechanical element 14.

In addition, moveable mechanical element 14 of MEMS sensor 10 isconfigured to receive electromagnetic input signals, such that whenelectromagnetic input signals are provided to the moveable mechanicalelement 14 of MEMS sensor 10, such as, for example, via the surfacecontact of pedestal 19, the moveable mechanical element 14 moves,responsive to the electromagnetic input signals, an amount correspondingto the magnitude and characteristics of the input signals. In anembodiment, MEMS sensor 10 is configured such that when an analogvoltage is applied to moveable mechanical element 14, the moveablemechanical element 14 of MEMS sensor 10 moves an amount that correspondsto the magnitude of the analog voltage, and continues to movecorresponding to any voltage changes in the input signals. In anembodiment, MEMS sensor 10 is configured such that when the moveablemechanical element 14 of MEMS sensor 10 moves, an analog voltagecorresponding to movements of the moveable mechanical portion 14 isprovided as an output of MEMS sensor 10 via sense contacts 18 and 9. Inan alternative embodiment, MEMS sensor 10 is configured such that whenthe moveable mechanical element 14 of MEMS sensor 10 moves, a signalother than voltage or capacitance corresponding to the movements of themoveable mechanical element 14 may be provided as an output of MEMSsensor 10 via sense contacts 18 and 9.

In an embodiment, MEMS sensor 10 is a MEMS accelerometer, providing anoutput signal corresponding to movement of the moveable mechanicalelement in response to acceleration or deceleration of MEMS sensor 10,or a device in which MEMS sensor 10 is present. In alternativeembodiments, MEMS sensor 10 may be any MEMS sensor configured as, forexample, an inertial sensor, gyroscope, pressure sensor, or any otherMEMS device configured to provide an output signal in response to aphysical stimulus, and capable of having an electromagnetic inputapplied to the moveable mechanical element or proof mass of the MEMSdevice. It should be appreciated that each MEMS device may have a numberof characteristics unique to each individual MEMS device, such as, forexample, a resonant frequency, damping characteristics, displacementcharacteristics, spring constant characteristics, thickness, spacebetween moveable mechanical elements, and other responsecharacteristics. It should also be appreciated that these uniquecharacteristics may change over time, and/or in response to physicalforces applied to the MEMS device or environmental conditions such astemperature. MEMS sensor 10 may be formed using existing and upcomingMEMS fabrication design rules and processes that include, for example,deposition, patterning, and etching. In an embodiment, MEMS sensor 10 isa MEMS sensor configured to respond to both physical and electromagneticstimuli by providing an output signal corresponding to the appliedstimulus.

FIG. 2 shows a block diagram of a sensor parameter identification andevaluation system 100 including the MEMS sensor 10 of FIG. 1 configuredin accordance with the teaching of an embodiment. Sensor parameteridentification and evaluation system 100 is configured to providesignals to a MEMS sensor 10 and determine various parameters of the MEMSsensor 10 based on the response of MEMS sensor 10 to the providedsignals. FIG. 2 omits some of the details of the MEMS sensor generallyillustrated in FIG. 1. Furthermore, although the MEMS sensor 10 of FIG.1 is illustrated having two sense contacts, 18 and 9, each associatedwith a separate capacitor of the MEMS sensor of FIG. 1, FIG. 2. onlyillustrates sense contact 18 associated with one of the capacitors. Itshould be appreciated that the operation with respect to sense contact 9and the second capacitor would be similar to the operation with respectto sense contact 18 and the first capacitor. In an alternativeembodiment, MEMS sensor 10 may have only one capacitor and once sensecontact.

Sensor parameter identification and evaluation system 100 includes gaincircuitry 60 electrically coupled to sense contact 18 of MEMS sensor 10.Gain circuitry 60 is configured to receive an electromagnetic signal 17from MEMS sensor 10 via sense contact 18, amplify the electromagneticsignal 17, and provide the amplified electromagnetic signal as amplifiedsignal 61 to gain circuitry 60. Electromagnetic signal 17 is asinusoidal amplitude-modulated signal comprising carrier and datacomponents. Electromagnetic signal 17 includes information that is inpart a function electromechanical characteristics of the MEMS sensor 10,and in part a function of input signals applied to MEMS sensor 10.

Sensor parameter identification and evaluation system 100 furthercomprises demodulator 70 electrically coupled to gain circuitry 60.Demodulator 70 is configured to receive amplified signal 61 as an inputfrom gain circuitry 60, process the received amplified signal 61 todemodulate amplified signal 61 into its component parts (removing thecarrier component), and provide as an output amplified demodulatedsignal 71. In an embodiment, demodulator 70 provides amplitudedemodulation, and is configured to demodulate amplitude-modulatedsignals provided to demodulator 70 via amplified signal 61 into anamplitude-modulated carrier component and a data component carried bythe carrier component, remove the amplitude-modulated carrier component,and to provide the demodulated data component as amplified demodulatedsignal 71.

Sensor parameter identification and evaluation system 100 furthercomprises low-pass filter 80 electrically coupled to demodulator 70.Low-pass filter 80 is a low-pass filter configured to receive thedemodulated data component, amplified demodulated signal 71, as an inputfrom demodulator 70, filter out higher frequency components fromamplified demodulated signal 71, and provide the resulting filteredsignal as an output, filtered demodulated signal 81. In an embodiment,low-pass filter 80 is configured to filter out high frequencies above500 kHz and pass low frequencies below 500 kHz.

Sensor parameter identification and evaluation system 100 also includessignal evaluation circuitry 50 electrically coupled to low-pass filter80 and other elements of sensor parameter identification and evaluationsystem 100. Signal evaluation circuitry 50 is configured to receivefiltered demodulated signal 81 from low-pass filter 80, and is furtherconfigured to receive at least one oscillating reference signal fromother elements of sensor parameter identification and evaluation system100 that are electrically coupled to signal evaluation circuitry 50.Signal evaluation circuitry 50 is further configured to determine theamplitude and phase of both filtered demodulated signal 81 and anoscillating reference signal. Signal evaluation circuitry 50 is stillfurther configured to compare the filtered demodulated signal 81 with anoscillating reference signal to determine the amplitude and phasedifference between the signals and provide this information as an outputvia signal 51.

Sensor parameter identification and evaluation system 100 also includesa processor 20 electrically coupled to signal evaluation circuitry 50and various other components of sensor parameter identification andevaluation system 100. In an alternative embodiment, processor 20 may beany type of processor, including a microcontroller or a state machine orany measurement circuitry used for the purpose of evaluating the signal51 and providing input signals. Processor 20 includes logic, and isshown having memory 22, and program 200 located in memory 22 andconfigured to cause processor 20 to perform various functions for sensorparameter identification and evaluation system 100. Memory 22 isnon-volatile memory. In an alternative embodiment, memory 22 may bevolatile memory. Memory 22 is further shown having calibration and/ortrim values associated with MEMS sensor 10 stored in memory 22. As shownin FIG. 2, processor 20 is electrically coupled to signal evaluationcircuitry 50 and digital-to-analog converter 30. Processor 20 isconfigured to generate sinusoidal digital waveform signals responsive toprogram 200, and provide those sinusoidal digital waveform signals todigital-to-analog converter 30 via signal 21. The digital waveformsignals include a high frequency component and a low frequencycomponent. In an embodiment, processor 20 includes a parameterdetermination and processing program 200 configured to identify andevaluate parameters of MEMS sensor 10 based in part on informationprovided by signal evaluation circuitry 50 via signal 51. Parameterdetermination and processing program 200 is shown stored in memory 22.In an alternative embodiment, parameter determination and processingprogram 200 may be stored in volatile or non-volatile memory locationsexternal to processor 20, or may be hard-wired into processor 20. In yetanother alternative embodiment, processor 20 may itself be a hard-wireddevice.

Processor 20 is configured, responsive to parameter determination andprocessing program 200, to monitor the operation of MEMS sensor 10 viasignal 51, provide sinusoidal input signals comprising both low andhigh-frequency oscillating components to MEMS sensor 10 to initiatemotion of the mechanical element 14 of MEMS sensor 10 responsive to thesinusoidal input signals, and to calculate, monitor, and evaluateparameters of MEMS sensor 10 based on the output of MEMS sensor 10provided by signal evaluation circuitry 50 via signal 51. Processor 20is further configured to determine parameters of MEMS sensor 10responsive to input signals provided by processor 20 to MEMS sensor 10via signal 21, using information provided by signal evaluation circuitry50 via signal 51 and characteristics of the input signals. Processor 20is further configured to perform calculations using the input signals,signal 51, and sensor parameters, to determine, based on thecalculations, if the MEMS sensor 10 is operating properly and/or ifadjustments need to be made in compensation and calibration valuesassociated with MEMS sensor 10 in sensor parameter identification andevaluation system 100. In an embodiment, the input signal 21, or aderivative of input signal 21, is used as a reference waveform by signalevaluation circuitry 50 to determine parameters of MEMS sensor 10. In anembodiment, processor 20 is further configured to utilize parameterinformation to calculate updated compensation and calibration values forMEMS sensor 10.

Sensor parameter identification and evaluation system 100 also comprisesdigital-to-analog converter 30 electrically coupled to processor 20.Digital-to-analog converter 30 is configured to receive sinusoidalsignal 21, comprising both high and low-frequency components, fromprocessor 20, convert signal 21 from a digital waveform signal to ananalog waveform signal, and provide the resulting analog signal waveformcomprising both high and low-frequency components external todigital-to-analog converter 30 via signal 31 to MEMS sensor 10 viapedestal 19.

Continuing with FIG. 2, sensor parameter identification and evaluationsystem 100 includes a low-pass filter 40 electrically coupled todigital-to-analog converter 30. Low-pass filter 40 is a low-pass filterconfigured to receive an analog waveform signal comprising both high andlow-frequency components, signal 31, as an input from digital-to-analogconverter 30, filter out higher frequency components from signal 31, andprovide the resulting filtered signal, which acts as a reference signal,as an output filtered reference signal 41. In an embodiment, low-passfilter 40 is configured to filter out high frequencies above 500 kHz andpass low frequencies below 500 kHz. Low-pass filter 40 is furtherelectrically coupled to signal evaluation circuitry 50, and isconfigured to provide filtered reference signal 41 to signal evaluationcircuitry 50 as a reference signal. Signal evaluation circuitry 50 isconfigured to receive filtered reference signal 41 from low-pass filter40, and is further configured to determine the amplitude and phase offiltered reference signal 41. Signal evaluation circuitry 50 is stillfurther configured to compare the filtered demodulated signal 81 withthe reference signal, filtered reference signal 41, to determine theamplitude and phase difference between the signals, and to provide thisinformation as an output via signal 51.

Returning to digital-to-analog converter 30 of FIG. 2, digital-to-analogconverter 30 is further shown electrically coupled to MEMS sensor 10. Asnoted above, digital-to-analog converter 30 is configured to receivesinusoidal signal 21 from processor 20, convert signal 21 from a digitalwaveform signal to an analog waveform signal, and provide the resultinganalog sinusoidal signal waveform external to digital-to-analogconverter 30 via signal 31. Digital-to-analog converter 30 is furtherconfigured to provide the resulting analog signal waveform, signal 31,to MEMS sensor 10 via pedestal 19, which will cause moveable mechanicalelement 14 of MEMS sensor 10 to move responsive to signal 31.

Referring to FIGS. 1 and 2, the operation of sensor parameteridentification and evaluation system 100, according to an embodiment ofthe invention in which parameters and characteristics of a MEMS sensor10 of sensor parameter identification and evaluation system 100 arebeing determined and evaluated, is generally described. As a preliminarymatter, it should be appreciated that typical moveable mechanicalelements of MEMS sensors exhibit both a mechanical movement andreactance response when subjected to a stimulus that is below a certainthreshold frequency, but exhibit only a reactance response whensubjected to a stimulus that is above that threshold frequency. Morespecifically, in response to higher frequencies, MEMS sensors act like acapacitor electrically, but do not typically exhibit physical movement.However, in response to lower frequencies, MEMS sensors typicallyexhibit physical/mechanical movement and also a reactance response. Inresponse to lower frequencies, the reactance response is typically smallenough that it is not detected by demodulator 70. It should also beappreciated that while each moveable mechanical element of MEMS sensorstypically has a different threshold frequency that is a function ofphysical properties of the moveable mechanical element, moveablemechanical elements and MEMS sensors having similar designcharacteristics will typically have similar threshold frequencies. Afrequency of less than 10 KHz would be considered a lower frequency, towhich a MEMS sensor would typically exhibit both a physical/mechanicaland reactance response. A frequency of greater than 500 KHz would beconsidered a higher frequency, to which a MEMS sensor would typicallyexhibit only a reactance response.

Responsive to parameter determination and processing program 200,processor 20 generates both a high-frequency digital waveform and alow-frequency digital waveform. The high-frequency digital waveform hasa frequency that is higher than the threshold frequency of moveablemechanical element 14 of MEMS sensor 10, and the low-frequency digitalwaveform has a frequency that is lower than the threshold frequency ofmoveable mechanical element 14 of MEMS sensor 10. In an embodiment, thehigh-frequency digital waveform has a frequency above 500 kHz, forexample 1 MHz, and the low-frequency digital waveform has a frequencyless than or equivalent to the mechanical bandwidth of the MEMS sensor10, for example between 1 Hz and 10 kHz. Processor 20 then combines thetwo digital waveforms by adding or summing the two waveforms, andprovides the combined digital waveform as an output digital waveformsignal, signal 21, to digital-to-analog converter 30. Digital-to-analogconverter 30 converts the digital waveform signal, signal 21, into ananalog waveform signal 31. It should be appreciated that analog waveformsignal 31 includes both a high-frequency component and a low-frequencycomponent. In an embodiment, the high-frequency component has afrequency above 500 KHz, for example 1 MHz, and the low-frequencycomponent has a frequency less than or equivalent to the mechanicalbandwidth of the MEMS sensor 10, for example between 1 Hz and 10 kHz.

Digital-to-analog converter 30 provides the analog waveform signal 31 asan input to low-pass filter 40. Low-pass filter 40 filters the receivedsignal 31 to remove the high-frequency component, and provides theremaining low-frequency component to signal evaluation circuitry 50 as areference low-frequency waveform. In an embodiment, the remaininglow-frequency component has a frequency less than or equivalent to themechanical bandwidth of the MEMS sensor 10, for example between 1 Hz and10 kHz.

Digital-to-analog converter 30 also provides signal 31 as an analogwaveform signal input to moveable mechanical element 14 of MEMS sensor10. MEMS sensor 10 responds to the applied signal 31 in two ways basedon the low-frequency and high-frequency components of signal 31. Inresponse to the low-frequency component of signal 31, moveablemechanical element 14 exhibits a movement response by physicallydisplacing relative to the substrate 12 of MEMS sensor 10. This resultsfrom the low-frequency component of signal 31 creating anelectromagnetic force that pulls the moveable mechanical portion 14 upand/or down relative to the substrate 12 of MEMS sensor 10, therebycausing the moveable mechanical portion 14 of MEMS sensor 10 to moveresponsive to the low-frequency component of signal 31. In addition toexhibiting a movement response to signal 31, MEMS sensor 10 alsoexhibits an electronic capacitive reactance response to thelow-frequency component of signal 31. In response to the high-frequencycomponent of signal 31, moveable mechanical element 14 does not exhibita movement response, but MEMS sensor 10 does exhibit a capacitivereactance response to the high-frequency component.

The physical movement of the moveable mechanical element 14 of MEMSsensor 10 relative to substrate 12 and sense contact 18 of MEMS sensor10 causes the capacitance at sense contact 18 of MEMS sensor 10 to varyin a manner that corresponds to the magnitude and frequency of themovement of the moveable mechanical element 14 of MEMS sensor 10. Inaddition, the capacitive reactance response of MEMS sensor 10 to boththe low and high-frequency components of signal 31 causes thecapacitance at sense contact 18 of MEMS sensor 10 to vary responsive tosignal 31. As a result, signal 17, the signal present at sense contact18 of MEMS sensor 10, is a function of, and corresponds to, both thephysical movement response of moveable mechanical element 14 to thelow-frequency component of signal 31, and the capacitive reactanceresponse of MEMS sensor 10 to the low and high-frequency components ofsignal 31. Because of the combination of both the low-frequency andhigh-frequency component effects, including the physical modulation ofmoveable mechanical element 14, signal 17 is in the form of anamplitude-modulated waveform.

Signal 17 is provided to gain circuitry 60, where signal 17 isamplified, with the amplified signal, amplified signal 61, being passedon to demodulator 70. As noted above, signal 17 is in the form of anamplitude-modulated waveform. Demodulator 70 processes signal 17 todemodulate signal 17 and remove the carrier wave portion of signal 17.In an embodiment, the carrier wave portion of signal 17 has a frequencybetween 500 KHz and 10 MHz. After demodulation, demodulator 70 providesthe demodulated signal, amplified demodulated signal 71, which includesboth high-frequency and low-frequency components, as an output. Low-passfilter 80 receives amplified demodulated signal 71, filters amplifieddemodulated signal 71 to remove the high-frequency component, andprovides the remaining low-frequency component as filtered demodulatedsignal 81.

Signal evaluation circuitry 50 receives filtered demodulated signal 81,which is the amplified, demodulated, low-frequency component of thesignal output at sense contact 18 of MEMS sensor 10. As noted above,signal evaluation circuitry 50 also receives signal 41, the filtered lowfrequency component of the signal 31 provided as a signal input tomoveable mechanical element 14 of MEMS sensor 10 (and sometimes referredto as a reference signal). Signal evaluation circuitry 50 compares theamplitude of filtered demodulated signal 81 with the amplitude of signal41 to determine the change in amplitude introduced into the input signal31 by MEMS sensor 10 (and referred to as the amplitude difference).Signal evaluation circuitry 50 also compares the phase of filtereddemodulated signal 81 with the phase of signal 41 to determine thedifference in phase introduced into the input signal 31 by MEMS sensor10 (referred to as the phase difference). Signal evaluation circuitry 50provides the amplitude and phase difference information to processor 20via output signal 51.

Responsive to parameter determination and processing program 200,processor 20 evaluates the amplitude and phase difference informationreceived from signal evaluation circuitry 50 via signal 51, anddetermines, based on calculations, various parameters of MEMS sensor 10.In an embodiment, processor 20 may use the amplitude and phasedifference information to determine various parameters of MEMS sensor10, such as, for example, resonant frequency, harmonic distortion,damping and frequency response. In an embodiment, processor 20 utilizesFast Fourier Transforms (FFTs) to evaluate the amplitude and phasedifference information to determine harmonic characteristics andparameters of MEMS sensor 10. Processor 20 may also use the amplitudeand phase difference information to determine the quality and theintegrity of the MEMS sensor 10, or if there is an impairment to thenormal operation of the MEMS sensor 10, for example abnormal operationdue to broken or non-responsive structure or foreign particle. Processor20 may also use the amplitude and phase difference information todetermine other parameters, such as, for example, spring constant,thickness of MEMS sensor 10, space between beams and/or capacitors onMEMS sensor 10, and other characteristics of MEMS sensor 10. In analternative embodiment, parameter determination and processing program200 in processor 20 first evaluates signal 51 received from signalevaluation circuitry 50 to calculate and/or estimate various parameterssuch as, for example, the etching bias thickness of the silicon of MEMSsensor 10, side slope of the MEMS sensor 10, critical dimension (CD) ofMEMS sensor 10, and fringe of MEMS sensor 10. Parameter determinationand processing program 200 then uses these parameters to calculateestimates of the mass, spring constant, and other properties of the MEMSsensor 10. Finally, parameter determination and processing program 200may use the amplitude and phase difference information and/or MEMSsensor 10 parameters determined in part by using the amplitude and phasedifference information, to determine trim, compensation and/orcalibration values associated with MEMS sensor 10. Application systemsusing MEMS sensor 10 may employ the determined trim values forcompensation and/or calibration, allowing MEMS sensor 10 output signalsto be properly utilized by the application. It should be appreciatedthat parameter determination and processing program 200 could beconfigured to periodically re-calculate the trim values for thecompensation and/or calibration, such that those values more accuratelyreflect the characteristics of MEMS sensor 10 as those characteristicsshift over time. It should also be appreciated that parameterdetermination and processing program 200 could be configured to storethe trim values for the compensation and/or calibration in a memorylocation for access and use by other devices and systems utilizing MEMSsensor 10.

In summary, by applying a known signal having both high-frequency andlow-frequency sinusoidal components to MEMS sensor 10, MEMS sensor 10 iscaused to provide as an output an amplitude-modulated waveformcontaining high-frequency and low-frequency components. Thelow-frequency components of this waveform are compared with the appliedknown signal to determine electromechanical parameters, such as resonantfrequency, harmonic distortion, and damping of the MEMS sensor 10through which the known signal has passed. These parameters may then beused by systems employing the MEMS sensor 10 to correlate output signalsprovided by MEMS sensor 10 with external forces to which the systems aresubjected. In an embodiment electrical-mechanical physical models and/orstatistical models established and verified by a representative data setof measurements of various MEMS sensor may be used to determine thefrequency above which MEMS sensors do not mechanically move responsiveto the applied frequency.

Once parameter determination and processing program 200 has determinedparameters of MEMS sensor 10, parameter determination and processingprogram 200 may store the determined parameters associated with MEMSsensor 10, including, for example, trim values for the compensationand/or calibration of MEMS sensor 10, in a memory location in a systememploying the MEMS sensor 10, or may compare the determined parametersto parameters already stored in the system and overwrite the storedvalues with the newly determined values if appropriate in order to keepthe MEMS sensor 10 properly compensated and/or calibrated. By usingupdated parameters, an accurate determination of the characteristics ofvarious stimuli applied to the MEMS sensor 10 may continue to be made inspite of changes to the physical and/or electromechanicalcharacteristics of the MEMS sensor 10 over time. These updated trimvalues for the compensation and/or calibration may be used byapplications utilizing the MEMS sensor 10 to properly adjust the outputsignals provided by MEMS sensor 10 to compensate for characteristics ofMEMS sensor 10, and changes in those characteristics over time. In anembodiment, parameter determination is performed on a MEMS sensor 10after the device has been manufactured, but before it has been placed inan application (in which case the elements of FIG. 2 except for MEMSsensor 10 itself may be implemented in a tester or other hardware deviceto which sense contact 18 and moveable mechanical element 14 of MEMSsensor 10 are electrically connected). Alternatively, parameterdetermination is performed on a MEMS sensor 10 after the device has beenplaced in an application, in which case the logic and memory necessaryto carry out the parameter testing (and generally illustrated in FIG. 2)may be present in the same application module or system in which MEMSsensor 10 is located. In this case, parameter determination can be madeat any time during the life of the system in which MEMS sensor 10 isoperating.

FIG. 3 shows a block diagram of a sensor parameter identification andevaluation system configured in accordance with the teaching of anotheralternative embodiment. In this embodiment, the elements generallyillustrated in FIG. 2, except for the MEMS sensor 10 itself, are shownimplemented in separate tester hardware 110. Tester hardware 110 iselectronically coupled to MEMS sensor 10 at sense contact 18 and themoveable mechanical element 14 via the surface contact for pedestal 19.It should be appreciated that tester hardware 110, when electricallycoupled to a MEMS sensor 10 and executing Program 200 as describedabove, functions in a similar fashion to the embodiment generallyillustrated in FIG. 2. In addition, in yet other alternativeembodiments, tester hardware 110 may be configured such that certain ofthe functions, such as, for example, signal filters 80 and 40,demodulator 70, gain circuitry 60, and signal evaluation circuitry 50,may be combined into one or more integrated circuit chips in tester 110,or may be implemented in software run on tester 110.

FIG. 4 shows a block diagram of a sensor parameter identification andevaluation system configured in accordance with the teaching of anotheralternative embodiment. In this alternative embodiment, the elementsgenerally illustrated in FIG. 2 are shown implemented along withadditional application circuitry in a stand-alone application module120. MEMs sensor 10 may be implemented on a separate integrated circuitfrom the rest of stand-alone application module 120. The circuitry otherthan the additional application circuitry operates as discussed abovewith respect to FIG. 2. The additional application circuitry isconfigured to allow processor 20 of stand alone application module 120,to operate in an application system, using information provided by theadditional circuitry and derived from MEMS sensor 10. The additionalapplication circuitry includes a sense contact 16 of MEMS sensor 10electrically coupled to capacitance-to-voltage circuitry 90 (referred toherein as C-to-V 90). C-to-V 90 is configured to receive an inputcapacitance from MEMS sensor 10 and convert it to a voltage output.C-to-V 90 is electrically coupled to analog-to-digital (A-to-D)conversion circuitry A-to-D 92. A-to-D 92 is configured to receive avoltage signal from C-to-V 56, convert the received analog voltagesignal to a digital signal representative of the analog voltage signal,and provide the digital voltage signal as a digital voltage signaloutput from A-to-D 92. A-to-D 92 is shown electrically coupled to abuffer 94. Buffer 94 is configured to store the digital voltage signalprovided by A-to-D 92, and to provide the digital voltage signal toprocessor 20 such that processor 20 may use the signal to monitor theoperation of MEMS sensor 10 and perform various programs and algorithmsutilizing the digital voltage signal.

In operation, the additional application circuitry is configured tomonitor, at sense contact 16 of MEMS sensor 10, an output capacitancesignal corresponding to the motion of moveable mechanical element 14 ofMEMS sensor 10. C-to-V 90 converts this capacitance signal to a voltage,and provides this output voltage signal to A-to-D 92. The output voltagesignal is buffered by buffer 94, and subsequently provided to processor20. It should be appreciated that the movable mechanical element 14 ofMEMS sensor 10 will be moving responsive to physical forces (stimulus)to which the MEMS sensor 10 is being subjected. In this case, thecapacitive output of MEMS sensor 10 will correspond to the motion of themoveable mechanical element 14 of MEMS sensor 10 responsive to thephysical stimulus. C-to-V 90 receives this capacitive output of MEMSsensor 10, converts it to a voltage, and provides the voltage signal tobuffer 94. Buffer 94 buffers the voltage signal and provides thebuffered signal to processor 20. Responsive to an Application Programrunning in processor 20, processor 20 evaluates the buffered signal, anddetermines, based on the signal (and using the trim values for thecompensation and/or calibration associated with MEMS sensor 10), howmuch the moveable mechanical element 14 of MEMS sensor 10 has moved. Thetrim values for the compensation and/or calibration have been determinedand stored previously, and may be used by applications employing MEMSsensor 10 to compensate for changes in characteristics of MEMS sensor 10over time. If the moveable mechanical element 14 of MEMS sensor 10 hasmoved beyond a predetermined amount, or has moved in a predeterminedpattern, processor 20 is configured to cause some action to be taken instand-alone application module 120, or in systems in which stand-aloneapplication module 120 may be present. For example, processor 20 maydetermine that a change in motion of, or forces applied to, theapplication module 120 or vehicles or structures in which applicationmodule 120 may be situated, has occurred. A change in motion couldcomprise acceleration, change in acceleration, velocity, rotation,pressure, or other forces or motion. Processor 20 may also determine themagnitude of such changes.

By utilizing one application module 120 to include both application andtesting functions, real-time testing and calibration of applicationmodules 120 may be performed while continuing to utilize the MEMS sensor10 in the target application, and without the need to disable the targetapplication or remove MEMS sensor 10 physically from the application fortesting. In an alternative embodiment, sense contacts 16 and 18 may be asingle contact electrically coupled to both C-to-V 90 and gain circuitry60.

FIG. 5 shows a flow chart of a sensor parameter identification andevaluation method 200, according to an embodiment. In an embodiment, themethod 200 is implemented by the execution of parameter determinationand processing program 200. In a first operation 202, a high-frequencydigital signal and a low-frequency digital signal are generated by aprocessor 20 (FIG. 1). The high-frequency signal is at a high enoughfrequency that application of the signal to MEMS sensor 10 will notcause the moveable mechanical element 14 of MEMS sensor 10 tomechanically move. In an embodiment, the high-frequency signal is at afrequency above 500 KHz, for example 1 MHz. The low-frequency signal isat a low enough frequency that application of the low-frequency signalto a MEMS sensor 10 will cause the moveable mechanical element 14 of theMEMS sensor 10 to mechanically move. In an embodiment, the low-frequencysignal is at a frequency less than or equivalent to the mechanicalbandwidth of the MEMS sensor 10, for example between 1 Hz and 10 kHz.

In a second operation 204, the high-frequency and low-frequency digitalsignals are combined into a single combined digital signal by adding orsumming the two signals. In a third operation 206, the combined digitalsignal is converted into a combined analog signal. In a fourth operation208, the combined analog signal is provided to MEMS sensor 10 and alow-pass signal filter. In a fifth operation 210, a modulated signal iscreated in the MEMS sensor 10 responsive to the combined analog signal.In a sixth operation 212, the modulated signal is detected at an outputof MEMS sensor 10.

In a seventh operation 214, the detected modulated signal is amplified.In eighth operation 216, the amplified, detected modulated signal isdemodulated to remove the carrier component of the signal. In a ninthoperation 218, the demodulated signal is filtered in a low-pass filterto remove high frequency components. In a tenth operation 220, thefiltered demodulated signal is evaluated and compared with the filteredcombined analog signal to determine the amplitude and phase differencesbetween the signals. In an eleventh operation 222, the amplitude andphase differences are provided to a processor. In a twelfth operation224, the amplitude and phase differences are processed to determineparameters of the MEMS sensor 10. In a thirteenth operation 226, thesystem utilizing the MEMS sensor 10 is updated with the new compensationand calibration parameters.

In an alternative embodiment (not shown), all of the componentsgenerally illustrated in FIG. 2 may be formed together on a singlesubstrate and provided as a unitary device. In yet another alternativeembodiment (not shown), all of the components generally illustrated inFIG. 2 may be coupled together in a single module.

Embodiments described herein provide for the identification ofparameters of MEMS devices without a need for applying physical forcesexternal to the MEMS device, and without a need for removing the MEMSdevice from systems in which it is operating. Systems and methods areprovided for testing MEMS devices and identifying MEMS device parametersthroughout the lifetime of those MEMS devices, even on a continuousbasis, while those devices remain located and functioning in systemsemploying the MEMS devices. Thus, systems and methods achieving designobjectives of low-cost MEMS parameter identification, repeated testingof MEMS sensors throughout the life of the MEMS sensors to identify MEMSparameters, testing of MEMS sensors without removal of the sensors fromthe application, and re-calibration of MEMS devices during the life ofthe MEMS devices are provided. The systems and methods further allow forreduced testing costs, real-time calibration, and improved reliabilityof the system utilizing the MEMS devices. The systems and methodsprovide for the testing and evaluation of the electromechanical responseof any MEMS device capable of receiving an electromagnetic signal intothe movable element of the MEMS device and providing an output signalindicative of the motion of the movable mechanical element of the MEMSdevice.

A microelectromechanical systems (MEMS) system comprising a MEMS sensor,a control circuit in electrical communication with the moveablemechanical element of the MEMS sensor, and demodulation circuitry inelectrical communication with the MEMS sensor and the control circuit.The MEMS system is configured to demodulate the MEMS output signalcorresponding to motion of the moveable mechanical element and providethe demodulated signal to the control circuit. The control circuit isconfigured to evaluate the demodulated signal to determine at least onecharacteristic of the MEMS sensor. Although the preferred embodiments ofthe invention have been illustrated and described in detail, it will bereadily apparent to those skilled in the art that various modificationsmay be made therein without departing from the spirit of the inventionor from the scope of the appended claims.

What is claimed is:
 1. A method of determining a parameter of a MEMSsensor in a system, comprising: generating an input signal including atleast one high-frequency component selected to not cause physicaloscillation of a moveable mechanical element of a MEMS sensor, and onelow-frequency component selected to cause physical oscillation of amoveable mechanical element of a MEMS sensor; providing the input signalto a MEMS sensor comprising at least one moveable mechanical element tocause a modulated output signal comprising high and low frequencycomponents to be provided from the MEMS sensor; de-modulating themodulated output signal provided by the MEMS sensor; filtering thede-modulated output signal to remove the high frequency components; and,evaluating the de-modulated, filtered signal to determine at least onecharacteristic of the MEMS sensor.
 2. A method as claimed in claim 1,wherein the step of evaluating the de-modulated output signal furtherincludes the step of comparing the de-modulated, filtered signal to theinput signal to determine at least one difference between the signals.3. A method as claimed in claim 2, wherein the at least one determineddifference includes at least one of amplitude and phase differences. 4.A method as claimed in claim 3, further including the step of utilizingthe at least one characteristic of the MEMS sensor to update at leastone of a compensation or calibration parameter for the MEMS sensor.
 5. Amethod as claimed in claim 1, wherein the steps are performed in anapplication module configured to perform functions other thanexclusively testing using an output from a MEMS sensor mounted in theapplication module, and wherein the module is configured to use outputfrom the MEMS sensor to determine a function performed by theapplication module.
 6. A method as claimed in claim 1, wherein the stepsare performed in equipment configured to temporarily electrically coupleto a various MEMS sensor for purposes of testing the MEMS sensors.
 7. Amethod as claimed in claim 1 wherein the low frequency component is afrequency between 1 kHz and 10 kHz, and wherein the at least one highfrequency component is a frequency greater than 500 kHz.
 8. A method asclaimed in claim 1 wherein the input signal is generated as a digitalsignal and converted to an analog signal prior to being provided to theMEMS sensor.
 9. A method as claimed in claim 1 further comprising: priorto the demodulating, amplifying with an amplifier the modulated outputsignal.
 10. A method as claimed in claim 1 wherein the generating andthe evaluating are performed by a control circuit executing code.
 11. Amethod as claimed in claim 2 further comprising: filtering the inputsignal to provide a filtered input signal, wherein the step of comparingthe de-modulated, filtered signal to the input signal to determine atleast one difference between the signals includes comparing thede-modulated, filtered signal to the filtered input signal.
 12. A methodas claimed in claim 11 wherein the filtering the input signal includesremoving the at least one high-frequency component selected to not causephysical oscillation of a moveable mechanical element of a MEMS sensorfrom the input signal.
 13. A method as claimed in claim 11 wherein theinput signal is generated as a digital signal and converted to an analogsignal prior to being provided to the MEMS sensor, wherein the filteringthe input signal includes filtering the input signal after it has beenconverted to an analog signal.
 14. A method as claimed in claim 1further comprising: converting an electromagnetic output of the MEMSsensor to a voltage signal; converting the voltage signal to a digitalvoltage signal; determining by a control circuit from the digitalvoltage signal, at least one of a change in motion of the MEMS sensor ora change in forces applied to the MEMS sensor.
 15. A method as claimedin claim 14 wherein the electromagnetic output and the modulated outputsignal are generated from a same output of the MEMS sensor.
 16. A methodas claimed in claim 1 further comprising: storing in a memory, the atleast one characteristic of the MEMS sensor determined from theevaluating.
 17. A method as claimed in claim 16 further comprising:overwriting at least one previously-stored characteristic with the atleast one characteristic of the MEMS sensor determined from theevaluating.
 18. A method as claimed in claim 1 wherein the de-modulatingthe modulated output signal includes removing a carrier wave portion ofthe modulated signal.
 19. A method of determining a parameter of a MEMSsensor in a system, comprising: generating an input signal including atleast one high-frequency component selected to not cause physicaloscillation of a moveable mechanical element of a MEMS sensor, and onelow-frequency component selected to cause physical oscillation of amoveable mechanical element of a MEMS sensor; providing the input signalto a MEMS sensor comprising at least one moveable mechanical element tocause an output signal comprising high and low frequency components tobe provided from the MEMS sensor; filtering the output signal to removethe high frequency components; filtering the input signal to provide afiltered input signal, wherein the filtering the input signal includesremoving the at least one high-frequency component; evaluating theoutput signal after the filtering the output signal to determine atleast one characteristic of the MEMS sensor, wherein the evaluating theoutput signal further includes comparing the output signal to thefiltered input signal to determine at least one difference between thesignals.
 20. A method as claimed in claim 19 wherein prior to filteringthe output signal, demodulating the output signal.